The Cardiac Cycle and Heart Function
Overview of Cardiac Function
- The heart consists of individual pacemaker cells and contractile cells that generate electrical signals allowing contraction.
- The heart functions as two pumps:
- Right Pump: Sends blood to the lungs through pulmonary circulation.
- Left Pump: Sends blood to the systemic circuit, delivering oxygenated blood to the body.
Pumping Mechanism
- The heart generates pressure necessary to propel blood throughout the body.
- Pressure must be sufficient to exceed the pressure in the pulmonary trunk (for the right ventricle) and in the aorta (for the left ventricle) to facilitate blood flow.
Ventricular Contraction and Relaxation
- Following contraction, the ventricles must relax to fill again.
- Systole: Period of contraction when blood is expelled from the heart.
- Diastole: Period of relaxation when heart chambers fill with blood.
Cardiac Cycle Phases
- The cardiac cycle consists of multiple phases:
- Passive Ventricular Filling
- Atrial Systole
- Ventricular Systole (broken into subphases)
- Ventricular Diastole
Passive Ventricular Filling
- When all chambers are relaxed, blood flows passively from:
- Superior and Inferior Vena Cava
- Pulmonary Veins
- Blood enters the right or left atrium due to higher pressure in these vessels.
- Atrioventricular (AV) valves (bicuspid and tricuspid) are open, allowing passive blood flow from atria to ventricles.
- Passive Ventricular Filling: Blood flows passively from the atria into the ventricles.
Atrial Systole
- Atrial Systole: Active ventricular filling occurs as the atria contract, facilitating the movement of blood into the ventricles.
- Initiated by electrical impulses from the SA node, generating an action potential that spreads to contractile cells.
- Atrial contraction corresponds to the P wave on an electrocardiogram (EKG).
- As the atria contract, they increase the volume of blood in the ventricles, known as the End Diastolic Volume (EDV), which is approximately 130 ext{ mL}.
Ventricular Systole
- Following atrial systole, the ventricles enter Systole, with two distinct phases:
- Isovolumetric Contraction
- Action potential travels down to the AV bundle and bundle branches, leading to increased ventricular pressure.
- Bicuspid and tricuspid valves close due to rising pressure; all valves are closed at this stage (no blood enters or exits).
- Ventricular pressure ultimately exceeds the pressure in the pulmonary trunk and the aorta, allowing for ejection of blood.
- Ejection Phase
- Semilunar valves (pulmonary and aortic) open as ventricular pressure exceeds vessel pressure, allowing blood to be ejected into the pulmonary trunk (right ventricle) and aorta (left ventricle).
- Amount of blood pumped out is referred to as Stroke Volume (SV), with typically 70 ext{ mL} of blood ejected (calculated from EDV - End Systolic Volume (ESV)).
- End Systolic Volume (ESV): Approximately 60 ext{ mL} remains in the ventricles post-contraction.
Ventricular Diastole
- Following the ventricular contraction, the heart goes into Ventricular Diastole:
- Ventricles relax and repolarize, represented by the T wave on EKG.
- Backflow of blood occurs from the pulmonary trunk and aorta due to falling pressure in the ventricles; however, this backflow is stopped by the closing of the semilunar valves.
Recap of Cardiac Cycle
- The sequence of events repeats in a cycle:
- Passive Ventricular Filling ➔ Atrial Systole ➔ Ventricular Systole ➔ Ventricular Diastole.
- Blood flows along pressure gradients:
- Always moves from areas of higher pressure to areas of lower pressure, critical for proper heart function and circulation.
Implications and Significance
- Understanding the dynamics of the cardiac cycle is fundamental for understanding heart physiology and pathology, as insufficient pressure generation could lead to inadequate blood circulation.
- Future topics will involve exploring the influence of systemic vessels on blood pressure as blood travels throughout the body, emphasizing the significance of pressure gradients.
